Liquid crystal displays are commonly used as display devices for compact electronic apparatuses, because they not only provide good quality images with little power but are also very thin. The liquid crystal molecules in a liquid crystal display do not emit any light themselves. The liquid crystal molecules have to be lit by a light source so as to clearly and sharply display text and images. Thus, a backlight module is generally needed for a liquid crystal display.
Referring to FIG. 6, a typical backlight module 6 includes a frame 60, a light guide plate 61, a light source 62, a reflective sheet 63, and an optical sheet assembly 64. The light guide plate 61 includes a light incident surface 610, a top surface 612 perpendicularly adjoining the light incident surface 610, and a bottom surface 614 opposite to the top surface 612. The light source 62 is disposed adjacent to the light incident surface 610. The reflective sheet 63 is disposed on the bottom surface 614. The optical sheet assembly 64 is disposed on the top surface 612. The frame 60 accommodates the light guide plate 61, the light source 62, the reflective sheet 63, and the optical sheet assembly 64.
The frame 60 includes a spacing board 602 extending from an inner wall (not labeled) thereof. The spacing board 602 has a generally T-shaped cross-section, and is for keeping the optical sheet assembly 64 in position relative to the top surface 612 of the light guide plate 61. Under normal conditions, a gap exists between the optical sheet assembly 64 and a portion of the spacing board 602 above the optical sheet assembly 64. The gap provides room for the optical sheet assembly 64 to expand during stability tests performed on the backlight module 6 or under high temperature conditions. However, because of the gap, the optical sheet assembly 64 is liable to slide or shift relative to the top surface 612. When this happens, the optical performance of the backlight module 6 may be degraded. Moreover, when the optical sheet assembly 64 is expanded, this may result in a so-called waving mura phenomena. When this happens, the optical performance of the backlight module 6 may be degraded.
What is needed, therefore, is a backlight module that can overcome the above-described deficiencies.